linux-kernel - Re: [PATCH v3 1/2] tracing: ring_buffer: Rewind persistent ring buffer when reboot

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Message-Id: <20250528092209.01fb26deee82e404d52a90c5@kernel.org>
Date: Wed, 28 May 2025 09:22:09 +0900
From: Masami Hiramatsu (Google) <mhiramat@...nel.org>
To: Steven Rostedt <rostedt@...dmis.org>
Cc: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
 linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org
Subject: Re: [PATCH v3 1/2] tracing: ring_buffer: Rewind persistent ring
 buffer when reboot

On Tue, 27 May 2025 09:47:57 -0400
Steven Rostedt <rostedt@...dmis.org> wrote:

> On Tue, 27 May 2025 12:54:44 +0900
> Masami Hiramatsu (Google) <mhiramat@...nel.org> wrote:
> 
> > Here is what I meant. As far as I ran my test, it looks good (it prevents
> > over-read by `cat per_cpu/cpu0/trace_pipe_raw`)
> > 
> > diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
> > index 5034bae02f08..de1831eb3446 100644
> > --- a/kernel/trace/ring_buffer.c
> > +++ b/kernel/trace/ring_buffer.c
> > @@ -5405,6 +5405,7 @@ rb_get_reader_page(struct ring_buffer_per_cpu *cpu_buffer)
> >         unsigned long flags;
> >         int nr_loops = 0;
> >         bool ret;
> > +       u64 ts;
> >  
> >         local_irq_save(flags);
> >         arch_spin_lock(&cpu_buffer->lock);
> > @@ -5423,6 +5424,18 @@ rb_get_reader_page(struct ring_buffer_per_cpu *cpu_buffer)
> >  
> >         reader = cpu_buffer->reader_page;
> >  
> > +       /*
> > +        * Now the page->commit is not cleared when it read.
> > +        * Check whether timestamp is newer instead. We also don't
> > +        * care the head_page is overwritten. In that case, timestamp
> > +        * should be newer than reader timestamp too.
> > +        */
> > +       ts = cpu_buffer->head_page->page->time_stamp;
> > +       if (ts < reader->page->time_stamp) {
> 
> Hmm, I think this test may be too fragile. The head_page can be moved
> by the writer, and this would need to handle races.

Good point! Can we pick the page out from ring buffer as same
as reader_page? If its timestamp is newer, we push the reader
page (swap reader and head), or push back the header page.

> 
> I found an issue with commit overflow and have a couple of bugs to fix that
> touches some of this code. Let's revisit after I get those fixed.

OK, let's review it.

BTW, we need a ring buffer test tool not depending on perf tool.

Thank you,

> 
> Thanks,
> 
> -- Steve
> 
> 
> 
> > +               reader = NULL;
> > +               goto out;
> > +       }
> > +
> >         /* If there's more to read, return this page */
> >         if (cpu_buffer->reader_page->read < rb_page_size(reader))
> >                 goto out;
> > 
> 


-- 
Masami Hiramatsu (Google) <mhiramat@...nel.org>